Re: [Chicken-users] the effect of set! on the top-level namespace

[Kon Lovett]

On Oct 15, 2007, at 7:42 AM, Terrence Brannon wrote:

> I would like to know:
> * what is happening to the symbol afunc in line 1 versus line 3. In
> particular is the same memory location being over-written?

Yes.

> * Also, in line 2 is first-func being set to a value which is not  
> destroyed by
> the set! call in line 3?

Yes.

> * I think I was anticipating that first-func would assume the behavior
> of second-func once afunc was re-defined.

No. Scheme is "by-value" not "by-name".

> * Finally, there is no need to make things complicated by using set!
> since the define version has the same behavior. But for some code I am
> writing personally, I have to use set! because define inside a let
> does not seem to affect the top-level the way I need it to.

Per Scheme spec.

The idiom is:

(define foo)
(let (foo-local-vars ...)
  foo-local-defs ...
  (set! foo (lambda ...)) )

> (set! afunc (lambda (x) (+ x 5)))      ;;; line 1
> (set! first-func afunc)
> (set! afunc (lambda (x) (* x 1000)))  ;;; line 3
> (set! second-func afunc)
>
> (first-func 1)
> (second-func 1)
>
> (afunc 1) ; same as second-func
>
> ;;; define version
> (define afunc (lambda (x) (+ x 5)))
> (define first-func afunc)
> (define afunc (lambda (x) (* x 1000)))
> (define second-func afunc)
>
> (first-func 1)
> (second-func 1)
>
> (afunc 1) ; same as second-func
>
>
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Best Wishes,
Kon